Generally, a 2-level power conversion circuit capable of outputting a binary voltage is used as the power conversion circuit in a power conversion apparatus.
The following are three problems inherent in the 2-level power conversion circuit. A first problem is that an output voltage contains a large quantity of higher harmonics, and a large harmonic filter is required for outputting preferable alternate current or direct current containing less of higher harmonic components. A second problem is that a large quantity of electromagnetic noises are caused with switching. A third problem is that an improvement of efficiency has a limit because of a large switching loss.
Solutions of the problems inherent in the 2-level power conversion circuit entail researches and developments of multilevel power conversion circuits capable of outputting a ternary or higher-valued voltage, and some of these multilevel power conversion circuits are in practical application. The multilevel power conversion circuit is capable of outputting a voltage waveform that is more approximate to the AC or DC as the level count increases, and the harmonic filter can be therefore downsized as compared with the 2-level power conversion circuit. The voltage applied to one main circuit switch element decreases, and hence it is feasible to reduce the electromagnetic noises and the switching loss.
In a power network system introducing a large amount of distributed power sources, the power conversion apparatuses are connected to respective power sources and loads to thereby prepare a tremendous number of power conversion apparatuses, resulting in a request for reducing the electromagnetic noises and the higher harmonics generated by the power conversion apparatuses to the greatest possible degree.
Accordingly, there is an increased expectation for a clean multilevel power conversion apparatus configured to increase the level count at a great rate above the 2-level power conversion circuit being a mainstream so far.
The multilevel power conversion circuit has, however, a problem that the circuit is configured by an increased number of elements. The increased number of elements lead to a rise in packaging difficulty and bring about an increase in cost and a decline of reliability.
The number of elements configuring the multilevel power conversion apparatus results in becoming larger with a larger stage count (level count) of the voltage to be output. A 3-phase/2-level power conversion circuit of a conventional type is configured by six main circuit switch elements, and, by contrast, each of 3-phase multilevel power conversion circuits of typically a cascade connection H-bridge type, a diode clamp type and a flying capacitor type requires a 6(n−1) number of main circuit switch elements against a level count n.
Each main circuit switch element requires a gate drive circuit, and hence there is a tremendous number of elements of peripheral circuits as the level count is increased. Patent document 1 proposes a method of reducing the number of elements configuring the gate drive circuit of the multilevel power conversion circuit. When a packaging method is improper, the power conversion circuit contains a parasitic inductance and a parasitic capacitance, resulting in causing an increase in electromagnetic noises. A consideration of heat radiation of the elements is also needed.